Method of producing silicon carbide

ABSTRACT

A method of producing silicon carbide is provided. The method includes heating a cured product of a curable silicone composition in a non-oxidizing atmosphere at a temperature exceeding 1,500° C. but not more than 2,600° C. The method is capable of producing high-purity silicon carbide simply and at a high degree of productivity, and is capable of simply producing a silicon carbide molded item having a desired shape and dimensions.

This application is a Divisional of co-pending application Ser. No.12/340,084, filed on Dec. 19, 2008 for which priority is claimed under35 U.S.C. §120 and under 35 U.S.C. §119(a) to Patent Application No.2007-338066 filed in Japan, file Dec. 27, 2007. The entire contents ofall of the above applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing silicon carbidethat is capable of producing high-purity silicon carbide simply and at ahigh degree of productivity using a silicone composition.

2. Description of the Prior Art

Silicon carbide ceramics are chemically stable at both normaltemperatures and high temperatures, and exhibit excellent mechanicalstrength at high temperature, and are therefore widely used ashigh-temperature materials. In recent years, in the field ofsemiconductor production, high-purity silicon carbide ceramic sinteredbodies, which exhibit excellent heat resistance and creep resistance,have started to be used as boards and process tubes and the like withinsteps for conducting wafer heat treatments or the thermal diffusion oftrace elements. If the silicon carbide material used in these stepscomprises impurity elements, then these impurity elements may becomeincorporated within the wafer during heating of the wafer, causingcontamination of the wafer. Accordingly, the silicon carbide materialused in these applications should preferably have as high a degree ofpurity as possible.

Known methods of producing silicon carbide powder include the Achesonprocess, silica reduction methods, and vapor phase reaction methods.However, silicon carbide produced using the Acheson process tends tosuffer from low purity, silica reduction methods suffer from problems ofuniformity resulting from heterogeneous mixing of the silica powder andthe silicon carbide powder, and vapor phase methods suffer from problemsof low productivity. Recently, a method that uses a silicon metal alloyas the starting raw material has been reported (see Patent Document 1),and although this method enables silicon carbide to be obtained at lowtemperatures, the steps are complex, including conducting reaction underhigh pressure. Further, methods of generating carbon-silicon bonds bymixing an ethyl silicate containing no carbon-silicon bonds with anorganic compound, and then heating and reacting the mixture have alsobeen reported (see Patent Documents 2 and 3), but the large quantity ofdecomposition products generated means it is difficult to claim thatthese methods offer a high degree of productivity.

Furthermore, because silicon carbide is usually resistant to sintering,conventionally, obtaining a silicon carbide molded item having a desiredshape and dimensions is far from simple.

-   [Patent Document 1] US 2006/0171873 A1-   [Patent Document 2] JP 11-171647 A-   [Patent Document 3] JP 2006-256937 A

SUMMARY OF THE INVENTION

An object of the present invention is to address the problems associatedwith the conventional technology described above, and provide aproduction method that is capable of producing high-purity siliconcarbide simply and at a high degree of productivity. Furthermore,another object of the present invention is to provide a productionmethod that is capable of simply producing a silicon carbide molded itemhaving a desired shape and dimensions.

As a result of intensive investigation aimed at addressing the problemsdescribed above, the inventors of the present invention discovered thatthe above objects could be achieved by a mineralization of a siliconecured product, and they were therefore able to complete the presentinvention.

In other words, the present invention provides a method of producingsilicon carbide, comprising heating a cured product of a curablesilicone composition in a non-oxidizing atmosphere at a temperatureexceeding 1,500° C. but not more than 2,600° C.

According to the production method of the present invention, because thestarting raw material is a silicone composition, purification can beperformed at the silicone composition stage, and by then simplythermally decomposing the silicone composition, a high-purity siliconcarbide molded item can be produced simply and at a high degree ofproductivity.

Furthermore, according to the production method of the presentinvention, by first preparing a silicone molded item having a desiredshape and dimensions, and then simply heating (namely, calcining) themolded item, a silicon carbide having a desired shape and dimensions canbe produced with comparative ease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the X-ray diffraction spectrum (a) and thepeak data thereof (b) for a yellow-green solid obtained in Example 1,together with the X-ray diffraction spectrum peak data for β-siliconcarbide crystals (c) (wherein, the vertical axes in the peak data graphsare logarithmic).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A more detailed description of the present invention is presented below.In this description, “room temperature” refers to the ambienttemperature, which can typically change within a range from 10 to 35° C.

—Curable Silicone Composition—

There are no particular restrictions on the curing mechanism for thecurable silicone composition used as the starting material in the methodof the present invention, and curable silicone compositions of anycuring type can be used. Examples include addition-curable,ultraviolet-curable, electron beam-curable and condensation-curablesilicone compositions.

The cured product of the curable silicone composition can be obtained byusing a conventional method to cure the composition in accordance withthe curing mechanism of the composition. In those cases where a siliconcarbide molded item of a desired shape and dimensions is required, thecomposition is preferably first molded into the desired shape having thedesired dimensions, and is then cured to obtain the cured product.

In those cases where the curable silicone composition needs to bemolded, the molding method employed can be selected from amongst castmolding, injection molding and extrusion molding methods and the like,in accordance with whether the composition is solid or liquid at roomtemperature. In the case of cast molding, the composition should beliquid at room temperature, and more specifically, preferably has aviscosity at room temperature within a range from 1 to 1,000,000 mPa·s,and even more preferably from 10 to 300,000 mPa·s.

In those cases where the curable silicone composition is molded, asilicon carbide powder may be added to the composition as an optionalcomponent to increase the strength of the resulting molded item. Thereare no particular restrictions on the particle shape of the addedsilicon carbide powder, but the volume average particle size of thesilicon carbide powder is preferably within a range from 0.01 to 10 μm,and is even more preferably from 0.02 to 1 μm. In this description, the“volume average particle size” refers to a value measured using a laserdiffraction and scattering particle size analyzer LA-920 (a productname, manufactured by Horiba, Ltd.), and represents the volume averageparticle size corresponding with 50% in a cumulative distribution. Asingle silicon carbide powder may be used, or two or more siliconcarbide powders that differ in terms of their volume average particlesizes or the like may be used in combination. Although there are noparticular restrictions on the silicon carbide powder, high-puritypowders are preferred, and for example, a powder obtained by using aconventional method to pulverize a silicon carbide obtained using theproduction method of the present invention may be used. If a siliconcarbide powder is added to the curable silicone composition, then thequantity added of the powder is preferably sufficient that the ratio atroom temperature of the silicon carbide powder relative to the entirecurable silicone composition is within a range from 25 to 80% by volume,and more preferably from 35 to 70% by volume.

As mentioned above, there are no particular restrictions on the curablesilicone composition, but of the various possible types of composition,addition-curable silicone compositions and condensation-curable siliconecompositions are preferred. These compositions are described below.

<Addition-Curable Silicone Composition>

The addition-curable silicone composition comprises, for example,

(a) an organopolysiloxane having at least two alkenyl groups bonded tosilicon atoms,

(b) an organohydrogenpolysiloxane having hydrogen atoms bonded tosilicon atoms, in which the molar ratio of the hydrogen atoms relativeto all the silicon atoms within a molecule is within a range from 0.2 to2.0, and

(c) a platinum group metal-based catalyst.

Component (a): Alkenyl Group-Containing Organopolysiloxane

The organopolysiloxane of the component (a) is the base polymer of theaddition-curable silicone composition, and contains at least two alkenylgroups bonded to silicon atoms. The component (a) may use either asingle organopolysiloxane, or a combination of two or more differentorganopolysiloxanes. Conventional organopolysiloxanes may be used as thecomponent (a). The weight average molecular weight of theorganopolysiloxane of the component (a), measured by gel permeationchromatography (hereafter abbreviated as “GPC”) and referenced againstpolystyrene standards, is preferably within a range from approximately300 to 10,000. Moreover, the viscosity at 25° C. of theorganopolysiloxane of the component (a) is preferably within a rangefrom 1 to 10,000 mPa·s, and is more preferably from approximately 10 to3,000 mPa·s. Provided the viscosity is within this range, the handlingproperties of the component (a) are favorable, and in those cases wherea silicon carbide powder is added to the composition, the compositioncan be readily mixed with the silicon carbide powder. From the viewpointof availability of the raw materials, the organopolysiloxane of thecomponent (a) is basically either a straight-chain structure containingno branching in which the molecular chain (the main chain) is composedof repeating diorganosiloxane units (R¹ ₂SiO_(2/2) units) and bothmolecular chain terminals are blocked with triorganosiloxy groups (R¹₃SiO_(1/2) units), or a cyclic structure containing no branching inwhich the molecular chain is composed of repeating diorganosiloxaneunits, although the structure may also partially include branchedstructures such as trifunctional siloxane units (R¹SiO_(3/2) units) orSiO_(4/2) units. (In the above formulas, R¹ represents identical ordifferent, unsubstituted or substituted monovalent hydrocarbon groups,preferably containing from 1 to 10 carbon atoms, and even morepreferably 1 to 8 carbon atoms.)

Examples of the component (a) include organopolysiloxanes containing atleast two alkenyl groups bonded to silicon atoms, represented by anaverage composition formula (1) shown below:

R¹ _(a)SiO_((4-a)/2)  (1)

(wherein, R¹ is as defined above, and a is a number that is preferablywithin a range from 1.5 to 2.8, more preferably from 1.8 to 2.5, andmost preferably from 1.95 to 2.05).

Examples of the monovalent hydrocarbon group represented by R¹ includealkyl groups such as a methyl group, ethyl group, propyl group,isopropyl group, butyl group, isobutyl group, tert-butyl group, pentylgroup, neopentyl group, hexyl group, octyl group, nonyl group or decylgroup; aryl groups such as a phenyl group, tolyl group, xylyl group ornaphthyl group; cycloalkyl groups such as a cyclopentyl group orcyclohexyl group; alkenyl groups such as a vinyl group, allyl group,propenyl group, isopropenyl group, butenyl group, hexenyl group oroctenyl group; cycloalkenyl groups such as a cyclohexenyl group; andgroups in which a portion of, or all of, the hydrogen atoms within oneof the above hydrocarbon groups have been substituted with a halogenatom such as a fluorine atom, bromine atom or chlorine atom, or with acyano group or the like, such as a chloromethyl group, chloropropylgroup, bromoethyl group, trifluoropropyl group or cyanoethyl group.

In the average composition formula (1), at least two of the R¹ groupsare alkenyl groups (and preferably alkenyl groups of 2 to 8 carbonatoms, and even more preferably 2 to 6 carbon atoms). In those caseswhere the organopolysiloxane of the component (a) has a straight-chainstructure, the alkenyl groups may be bonded solely to silicon atoms atthe molecular chain terminals, solely to silicon atoms at non-terminalpositions within the molecular chain, or may also be bonded to boththese types of silicon atoms. In terms of achieving a favorable curingrate for the composition and superior physical properties for the curedproduct, at least one alkenyl group is preferably bonded to a siliconatom at a molecular chain terminal.

Basically, the R¹ groups may be any of the groups listed above, althoughthe alkenyl groups are preferably vinyl groups, and the monovalenthydrocarbon groups other than the alkenyl groups are preferably methylgroups or phenyl groups.

Specific examples of the component (a) include compounds represented bythe general formulas shown below.

In the above general formulas, R has the same meaning as R¹ with theexception of not including alkenyl groups. b and c are integers thatsatisfy b≧0 and c≧1 respectively, provided that b+c is a number thatyields a weight average molecular weight and a viscosity for theorganopolysiloxane that fall within the ranges specified above (namely,from 300 to 10,000, and from 1 to 10,000 mPa·s, preferably fromapproximately 10 to 3,000 mPa·s).

Component (b): Organohydrogenpolysiloxane

The organohydrogenpolysiloxane of the component (b) comprises sufficientsilicon atom-bonded hydrogen atoms (namely, SiH groups) that the molarratio of SiH groups relative to the total number of silicon atoms withinthe molecule is within a range from 0.2 to 2.0, is preferably at least0.2 and less than 2.0, is more preferably within a range from 0.5 to1.5, and is most preferably from 0.7 to 1.0. If this molar ratio is lessthan 0.2, then the mechanical strength following calcination of thecomposition under a non-oxidizing atmosphere tends to be inferior,whereas if the molar ratio exceeds 2.0, then production of theorganohydrogenpolysiloxane becomes difficult, the versatilitydiminishes, and the composition becomes economically unviable. Thecomponent (b) reacts with the component (a) and functions as across-linking agent. The component (b) may use either a singleorganohydrogenpolysiloxane, or a combination of two or more differentorganohydrogenpolysiloxanes.

There are no particular restrictions on the molecular structure of thecomponent (b), and conventionally produced chain-like, cyclic, branched,or three dimensional network (resin-like) organohydrogenpolysiloxanescan be used. If the component (b) has a chain-like structure, then theSiH groups may be bonded solely to silicon atoms at the molecular chainterminals, solely to silicon atoms at non-terminal positions within themolecular chain, or may also be bonded to both these types of siliconatoms. Furthermore, the number of silicon atoms within a single molecule(namely, the polymerization degree) is typically within a range from 2to 300 and is preferably from 4 to 150. An organohydrogenpolysiloxanethat is liquid at room temperature is particularly favorable as thecomponent (b).

Examples of the component (b) include organohydrogenpolysiloxanesrepresented by an average composition formula (2) shown below.

R² _(d)H_(e)SiO_((4-d-e)/2)  (2)

(wherein, R² represents identical or different, unsubstituted orsubstituted monovalent hydrocarbon groups, preferably containing from 1to 10 carbon atoms, and even more preferably from 1 to 8 carbon atoms, dand e represent numbers that preferably satisfy 0.7≦d≦2.1, 0.001≦e≦1.0and 0.8≦d+e≦3.0, and even more preferably satisfy 1.0≦d≦2.0, 0.01≦e≦1.0and 1.5≦d+e≦2.5)

Examples of R² include the same groups as those listed above for R¹within the above average composition formula (1) (but excluding alkenylgroups).

Specific examples of organohydrogenpolysiloxanes represented by theabove average composition formula (2) include1,1,3,3-tetramethyldisiloxane, 1,3,5,7-tetramethylcyclotetrasiloxane,tris(hydrogendimethylsiloxy)methylsilane,tris(hydrogendimethylsiloxy)phenylsilane,methylhydrogencyclopolysiloxane, cyclic copolymers ofmethylhydrogensiloxane and dimethylsiloxane, methylhydrogenpolysiloxanewith both terminals blocked with trimethylsiloxy groups, copolymers ofmethylhydrogensiloxane and dimethylsiloxane with both terminals blockedwith trimethylsiloxy groups, dimethylpolysiloxane with both terminalsblocked with dimethylhydrogensiloxy groups, copolymers ofmethylhydrogensiloxane and dimethylsiloxane with both terminals blockedwith dimethylhydrogensiloxy groups, copolymers of methylhydrogensiloxaneand diphenylsiloxane with both terminals blocked with trimethylsiloxygroups, copolymers of methylhydrogensiloxane, diphenylsiloxane anddimethylsiloxane with both terminals blocked with trimethylsiloxygroups, copolymers of methylhydrogensiloxane, methylphenylsiloxane anddimethylsiloxane with both terminals blocked with trimethylsiloxygroups, copolymers of methylhydrogensiloxane, diphenylsiloxane anddimethylsiloxane with both terminals blocked with dimethylhydrogensiloxygroups, copolymers of methylhydrogensiloxane, methylphenylsiloxane anddimethylsiloxane with both terminals blocked with dimethylhydrogensiloxygroups, copolymers composed of (CH₃)₂HSiO_(1/2) units, (CH₃)₂SiO_(2/2)units and SiO_(4/2) units, copolymers composed of (CH₃)₂HSiO_(1/2) unitsand SiO_(4/2) units, and copolymers composed of (CH₃)₂HSiO_(1/2) units,SiO_(4/2) units, and (C₆H₅)₃SiO_(1/2) units.

The quantity added of the component (b) must be sufficient that thequantity of SiH groups within this component (b), per 1.0 mols ofsilicon atom-bonded alkenyl groups within the component (a), is within arange from 0.1 to 5.0 mols, preferably from 0.5 to 3.0 mols, and evenmore preferably from 0.8 to 2.0 mols. Provided the quantity added of thecomponent (b) yields a quantity of SiH groups within the above range,the curing of the curable silicone composition is more likely to proceedsatisfactorily.

Component (c): Platinum Group Metal-Based Catalyst

The platinum group metal-based catalyst of the component (c) is used asa catalyst for accelerating the addition curing reaction (thehydrosilylation reaction) between the component (a) and the component(b). The component (c) may use either a single catalyst, or acombination of two or more different catalysts. Conventional platinumgroup metal-based catalysts can be used as the component (c), althoughthe use of platinum or a platinum compound is preferred. Specificexamples of the component (c) include platinum black, platinic chloride,chloroplatinic acid, alcohol-modified products of chloroplatinic acid,complexes of chloroplatinic acid with olefins, aldehydes, vinylsiloxanesor acetylene alcohols, and complexes of platinum and vinylsiloxanes.Other conventional platinum group metal-based catalysts that are usedfor addition curing reactions (hydrosilylation reactions) can also beused.

The quantity added of the component (c) need only be an effectivecatalytic quantity, may be suitably increased or decreased in accordancewith the desired curing rate, and preferably yields an equivalent massof the platinum group metal relative to the mass of the component (a)that is within a range from 0.1 to 1,000 ppm, and even more preferablyfrom 1 to 200 ppm.

Composition Preparation

The addition-curable silicone composition can be prepared by mixing thecomponents (a) to (c) using a conventional method. From the viewpointsof the molding and handling properties of the composition, the viscosityof the composition at room temperature is preferably within a range from10 to 200,000 mPa·s, and even more preferably from 50 to 100,000 mPa·s.

Composition Curing

Curing of the addition-curable silicone composition can be conductedusing conventional methods. In other words, curing is typicallyperformed either by leaving the composition to stand at room temperaturefor a long period, or by subjecting the composition to heating, therebycausing a hydrosilylation reaction to proceed within the composition.

Because the curing rate varies depending on the makeup of thecomposition, conditions such as the heating temperature may be selectedappropriately in accordance with the blend quantities of the respectivecomponents. The heating temperature is typically within a range fromroom temperature to 300° C., and a temperature within a range from 50 to200° C. is often satisfactory. The curing time may be set as desiredwithin a range from 1 minute to 3 hours, and is preferably from 3minutes to 2 hours. Furthermore, secondary curing may be conducted ifrequired, and if conducted, the temperature conditions for the secondarycuring are typically 120° C. or higher, and are frequently within arange from 150 to 250° C. The secondary curing time is typically withina range from 10 minutes to 48 hours, and a time of 30 minutes to 24hours is often satisfactory.

<Condensation-Curable Silicone Composition>

The condensation-curable silicone composition comprises, for example,

(α) a silicone resin represented by an average composition formula (3)shown below:

R³ _(m)R⁴ _(n)(OR⁵)_(p)(OH)_(q)SiO_((4-m-n-p-q)/2)  (3)

(wherein, each R³ represents, independently, a hydrogen atom or amonovalent hydrocarbon group other than an aryl group that eithercontains or does not contain a carbonyl group,

R⁴ represents a phenyl group,

R⁵ represents a monovalent hydrocarbon group of 1 to 4 carbon atoms,

m represents a number that satisfies: 0.1≦m≦2,

n represents a number that satisfies: 0≦n≦2,

p represents a number that satisfies: 0≦p≦1.5, and

q represents a number that satisfies: 0≦q≦0.35,

provided that a value of m+n+p+q satisfies: 0.1≦m+n+p+q≦2.6),

(β) a hydrolyzable silane or a partial hydrolysis-condensation productthereof or a combination thereof as an optional component, and

(γ) a condensation reaction catalyst as an optional component.

Component (α): Silicone Resin

The silicone resin used as the component (α) is a silicone resinrepresented by the above average composition formula (3). Here, the term“silicone resin” refers to an organopolysiloxane that adopts a threedimensional structure as a result of containing T units (trifunctionalsiloxane units) and/or Q units (tetrafunctional siloxane units). In somecases, the silicone resin may also include M units (monofunctionalsiloxane units) and/or D units (difunctional siloxane units).

The silicone resin used as the component (α) is preferably a solid, atleast at temperatures of room temperature or lower, and particularly attemperatures of 25° C. or lower, and has a softening point that ispreferably 40° C. or higher, and even more preferably within a rangefrom 40 to 100° C.

Next is a description of the above average composition formula (3) thatrepresents the component (α).

In the formula (3), each of the R³ groups preferably represents,independently, either a hydrogen atom, or a monovalent hydrocarbon groupother than an aryl group that contains from 1 to 8 carbon atoms andeither contains or does not contain a carbonyl group. Specific examplesof R³ include a hydrogen atom; alkyl groups such as a methyl group,ethyl group, propyl group, butyl group, pentyl group or hexyl group;cycloalkyl groups such as a cyclopentyl group or cyclohexyl group;alkenyl groups such as a vinyl group, allyl group, propenyl group,isopropenyl group or butenyl group; and acyl groups such as an acryloylgroup or methacryloyl group. From the viewpoint of ease of availabilityof the raw materials, R³ is preferably a hydrogen atom, or a methylgroup, ethyl group or vinyl group. In those cases where R³ is a hydrogenatom, the reactive SiH groups that exist within the silicone resinimprove the reactivity with the oxide coating (silica) that typicallycovers the surface of the silicon carbide powder that may be added tothe condensation-curable silicone composition as an optional component.

The aforementioned m is a number that satisfies: 0.1≦m≦2, the upperlimit for m is preferably 1.5 or lower, and the lower limit for m ispreferably at least 0.1, and even more preferably 0.5 or higher.Provided the value of m falls within this range, the fluidity of thesilicone resin is more readily reduced, meaning a uniform mixture withthe optional silicon carbide powder can be achieved more readily at acomparatively low temperature. Furthermore, because any reduction in themass of the mineralized product obtained by conducting a heat treatmentfollowing the curing can be more readily suppressed, resources can bebetter conserved, which is also economically more advantageous.

The aforementioned R⁴ group is a phenyl group, and in those cases wherea silicon carbide powder is added to the condensation-curable siliconecomposition, is useful in improving the wettability relative to thesilicon carbide powder.

The aforementioned n is a number that satisfies: 0≦n≦2, the upper limitfor n is preferably 1.5 or lower, and the lower limit for n ispreferably at least 0.05, and even more preferably 0.1 or higher.Provided the value of n falls within this range, the phenyl groupcontent is not too high, and because any reduction in the mass of themineralized product obtained by conducting a heat treatment followingcuring can be more readily suppressed, resources can be betterconserved, which is also economically more advantageous.

Specific examples of R⁵ include alkyl groups of 1 to 4 carbon atoms suchas a methyl group, ethyl group, propyl group, isopropyl group, butylgroup or isobutyl group, and a methyl group is particularly preferredindustrially. If the number of carbon atoms within R⁵ exceeds 4, thenthe reactivity of the group represented by OR⁵ becomes overly poor,which may result in deformation in the shape of the mineralized productduring the heat treatment conducted following curing.

The aforementioned p is a number that satisfies: 0≦p≦1.5, the upperlimit for p is preferably 1.2 or lower, and the lower limit for p ispreferably at least 0.05 and even more preferably 0.1 or higher.Provided the value of p falls within this range, the quantity within thesilicone resin of the group represented by OR⁵ is not too high, and themolecular weight of the silicone resin can be maintained at a highvalue, meaning any loss of carbon or silicon from the material as aresult of elimination and gasification during the heat treatmentconducted following curing can be suppressed to low levels.

The aforementioned q is a number that satisfies: 0≦q≦0.35, and ispreferably a number that satisfies: 0≦q≦0.3, and is most preferably 0.The value of q represents the small quantity of residual silanol groupsthat may be retained within the silicone resin during production.Provided the value of q falls within the above range, the reactivity ofthe silanol groups can be suppressed for the silicone resin as a whole,and both the storage stability and workability of the silicone resin canbe more readily improved.

The value of m+n+p+q is a number that satisfies: 0.1≦m+n+p+q≦2.6. Avalue of m+n+p+q within this range has the effect of enabling more readysuppression of any loss of carbon or silicon from the material as aresult of elimination and gasification during the heat treatmentconducted following curing.

There are no particular restrictions on the molecular weight of thesilicone resin, provided it is sufficient to enable ready suppression ofany loss of carbon or silicon from the material as a result ofelimination and gasification during the heat treatment conductedfollowing curing. For example, the weight average molecular weight ofthe silicone resin, measured by GPC and referenced against polystyrenestandards, is preferably at least 600, and is even more preferablywithin a range from 1,000 to 10,000.

There are no particular restrictions on the silicone resin provided itsatisfies the conditions described above. The silicone resin of thecomponent (a) may be either a single resin, or a combination of two ormore resins with different molecular structures or different proportionsof the various siloxane units.

These types of silicone resins can be produced by conventional methods.For example, the target silicone resin can be produced by conducting acohydrolysis, if required in the presence of an alcohol of 1 to 4 carbonatoms, of the organochlorosilanes that correspond with the siloxaneunits incorporated within the structure of the target resin, using aratio between the organochlorosilanes that reflects the ratio betweenthe corresponding siloxane units, while removing the by-producthydrochloric acid and low boiling point components. Furthermore, inthose cases where alkoxysilanes, silicone oils or cyclic siloxanes areused as starting raw materials, the target silicone resin can beobtained by using an acid catalyst such as hydrochloric acid, sulfuricacid or methanesulfonic acid, adding water to effect the hydrolysis ifrequired, and following completion of the polymerization reaction,removing the acid catalyst and low boiling point components.

Component (β): Hydrolyzable Silane or Partial Hydrolysis-CondensationProduct Thereof or Combination Thereof.

The hydrolyzable silane or partial hydrolysis-condensation productthereof or combination thereof that functions as the component (β) actsas a curing agent, but is an optional component that need notnecessarily be used. The component (β) may use either a single materialor a combination of two or more different materials. Examples ofcompounds that can be used favorably as the component (β) includesilanes containing at least three silicon atom-bonded hydrolyzablegroups within each molecule, partial hydrolysis-condensation productsthereof (namely, organopolysiloxanes in which at least one, andpreferably two or more, of the hydrolyzable groups remain within themolecule), and combinations thereof. In this description, the term“hydrolyzable group” describes a group that can generate a hydroxylgroup upon hydrolysis under the action of water.

Examples of compounds that can be used favorably as the abovehydrolyzable silane include compounds represented by a formula (4) shownbelow:

R⁶ _(f)SiX_(4-f)  (4)

(wherein, R⁶ represents an unsubstituted or substituted monovalenthydrocarbon group, X represents a hydrolyzable group, and f representseither 0 or 1).

Preferred examples of R⁶ include alkyl groups such as a methyl group orethyl group; alkenyl groups such as a vinyl group, allyl group orpropenyl group; and aryl groups such as a phenyl group. Examples of Xinclude acyloxy groups such as an acetoxy group, octanoyloxy group orbenzoyloxy group; ketoxime groups (namely, iminoxy groups) such as adimethyl ketoxime group, methyl ethyl ketoxime group or diethyl ketoximegroup; alkoxy groups such as a methoxy group, ethoxy group or propoxygroup; alkoxyalkoxy groups such as a methoxyethoxy group, ethoxyethoxygroup or methoxypropoxy group; alkenyloxy groups such as a vinyloxygroup, isopropenyloxy group or 1-ethyl-2-methylvinyloxy group; aminogroups such as a dimethylamino group, diethylamino group, butylaminogroup or cyclohexylamino group; aminoxy groups such as a dimethylaminoxygroup or diethylaminoxy group; and amide groups such as anN-methylacetamide group, N-ethylacetamide group or N-methylbenzamidegroup.

Specific examples of the component (β) include hydrolyzable silanes suchas methyltriethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilaneand ethyl orthosilicate; and partial hydrolysis-condensation products ofthese compounds.

In those cases where a hydrolyzable silane, a partialhydrolysis-condensation product thereof, or a combination thereof isused as the component (β), the quantity added is preferably within arange from 0.01 to 20 parts by mass, and even more preferably from 0.1to 10 parts by mass, per 100 parts by mass of the silicone resin of thecomponent (α). In those cases where the component (β) is used, using aquantity within the above range ensures that the storage stability,adhesion and curing rate of the composition of the present invention areparticularly favorable.

Component (γ): Condensation Reaction Catalyst

The condensation reaction catalyst of the component (γ) is an optionalcomponent, and need not necessarily be added. In those cases where theabove hydrolyzable silane or partial hydrolysis-condensation productthereof or combination thereof of the component (β) contains aminoxygroups, amino groups or ketoxime groups or the like, the component (γ)need not be used. The component (γ) may use either a single catalyst ora combination of two or more different catalysts. Examples of thecondensation reaction catalyst of the component (γ) includeorganotitanate esters such as tetrabutyl titanate and tetraisopropyltitanate; organotitanium chelate compounds such asdiisopropoxybis(acetylacetonato)titanium anddiisopropoxybis(ethylacetoacetate)titanium; organoaluminum compoundssuch as aluminum tris(acetylacetonate) and aluminumtris(ethylacetoacetate); organozirconium compounds such as zirconiumtetra(acetylacetonate) and zirconium tetrabutyrate; organotin compoundssuch as dibutyltin dioctoate, dibutyltin dilaurate and dibutyltindi(2-ethylhexanoate); metal salts of organic carboxylic acids such astin naphthenate, tin oleate, tin butyrate, cobalt naphthenate and zincstearate; amine compounds or the salts thereof such as hexylamine anddodecylamine phosphate; quaternary ammonium salts such asbenzyltriethylammonium acetate; lower aliphatic acid salts of alkalimetals such as potassium acetate; dialkylhydroxylamines such asdimethylhydroxylamine and diethylhydroxylamine; and guanidylgroup-containing organosilicon compounds.

In those cases where a condensation reaction catalyst of the component(γ) is used, although there are no particular restrictions on thequantity added, the quantity is preferably within a range from 0.01 to20 parts by mass, and even more preferably from 0.1 to 10 parts by mass,per 100 parts by mass of the component (α). If the component (γ) isused, then provided the quantity falls within the above range, thecurability and storage stability of the composition of the presentinvention are particularly favorable.

In the condensation-curable silicone composition described above, thequantities of the component (β) and the component (γ) are preferablykept as small as possible, and compositions that do not use thesecomponents are particularly desirable. In other words, preferredcondensation-curable silicone compositions are composed essentially ofonly the component (α), or only the component (α) and a silicon carbidepowder, as such compositions exhibit more favorable retention of theshape and dimensions of the molded item over the course of the heattreatment. Here the expression “composed essentially of only thecomponent (α), or only the component (α) and a silicon carbide powder”means that the incorporation of very small quantities of unavoidableother components is permissible, but incorporation of significantquantities of these other components is excluded.

Composition Preparation

In those case where the condensation-curable silicone compositioncomprises the component (β), the component (γ) and/or a silicon carbidepowder, the composition can be prepared by mixing the component (β), thecomponent (γ) and/or the silicon carbide powder with the component (α),which may be in a solventless state, namely those cases where thecomponent (α) is a liquid at room temperature, a liquid state formed byheating and melting the solid component (α), a water-based emulsionstate obtained by emulsifying the component (α) in water, or an organicsolvent solution state obtained by dissolving the component (α) in anorganic solvent such as toluene or tetrahydrofuran.

In those cases where the component (α) is a liquid at room temperature,and is simply mixed with the other components in a solventless state,the viscosity of the component (α) at 25° C. is preferably within arange from 1 to 50,000 mPa·s, and more preferably from 10 to 10,000mPa·s. In those cases where the component (α) is a solid at roomtemperature, and is heated and melted to generate a liquid state beforemixing with the other components, the heating temperature is preferablyset so as to achieve a viscosity for the liquid component (α) within arange from 1 to 50,000 mPa·s, and more preferably from 10 to 10,000mPa·s. In those cases where the component (α) is converted to awater-based emulsion or an organic solvent solution before mixing withthe other components, the viscosity at 25° C. of the water-basedemulsion or organic solvent solution is preferably adjusted to a valuewithin a range from 1 to 50,000 mPa·s, and more preferably from 10 to10,000 mPa·s.

If the condensation-curable silicone composition is a liquid at roomtemperature, then from the viewpoints of the molding and handlingproperties of the composition, the viscosity of the composition at roomtemperature is preferably within a range from 10 to 200,000 mPa·s, andeven more preferably from 50 to 100,000 mPa·s. If the composition is asolid at room temperature, then from the viewpoints of the molding andhandling properties of the composition, the viscosity of the liquidstate obtained by heating and melting the composition is preferably setto a value within a range from 10 to 200,000 mPa·s, and even morepreferably from 50 to 100,000 mPa·s.

Composition Curing

If the condensation-curable silicone composition is left to stand withinan atmosphere that contains moisture (for example, a humidity within arange from 25 to 90% RH, and preferably from 50 to 85% RH), thecomposition cures under the action of the moisture in the atmosphere.Heating at a temperature of 300° C. or lower (for example, a temperaturefrom 40 to 300° C.) may be used to accelerate the curing of thecomposition. Furthermore, secondary curing may also be conducted ifrequired, and the temperature conditions during such secondary curingare preferably at least 120° C., and more preferably within a range from150 to 250° C. The secondary curing time is preferably within a rangefrom 10 minutes to 48 hours, and is even more preferably from 30 minutesto 24 hours.

In those cases where the silicone composition comprises the component(α) and the component (β), but contains no condensation catalyst of thecomponent (γ), the composition is subjected to a heat treatment. Thisheat treatment causes the condensation reaction within the compositionto proceed, thereby curing the composition. Because the curing ratevaries depending on the quantity of the silicone resin within thecomposition, the temperature conditions for curing may be selectedappropriately in accordance with this quantity of the silicone resin,but the heating temperature is preferably within a range from 100 to300° C., and even more preferably from 150 to 250° C. The curing time istypically within a range from 1 minute to 3 hours, and is preferablyfrom 3 minutes to 2 hours. Furthermore, secondary curing may beconducted if required, and the conditions for such secondary curing areas described above.

—Mineralization of the Silicone Cured Product—

The cured product of the curable silicone composition is subjected to aheat treatment in a non-oxidizing atmosphere, thereby causingmineralization of the silicone.

This heat treatment is conducted under a non-oxidizing atmosphere, andpreferably under an inert gas atmosphere. Examples of the inert gasinclude nitrogen gas, argon gas and helium gas, although in order toachieve a higher purity silicon carbide, argon gas is particularlydesirable.

The heat treatment is conducted at a temperature exceeding 1,500° C. butnot more than 2,600° C. This heating temperature is preferably 1,600° C.or higher. Furthermore, the heating temperature is preferably not morethan 2,100° C., and is even more preferably 2,000° C. or lower. In otherwords, the heating temperature is preferably within a range from 1,600to 2,100° C., and more preferably from 1,600 to 2,000° C. This heattreatment firstly causes cleavage of carbon-hydrogen bonds within thesilicone and elimination of hydrogen from within the material attemperatures within a range from 400 to 1,500° C., although themineralization proceeds without elimination of silicon and carbon.Within this temperature range, large quantities of oxygen remain withinthe inorganic ceramic material produced by the mineralization,indicating that the temperature is insufficient for producing siliconcarbide. When the temperature exceeds 1,500° C., elimination of carbonmonoxide begins to occur, eventually leading to the formation of siliconcarbide. If the temperature exceeds 2,600° C., then the level of siliconcarbide sublimation becomes overly severe.

Furthermore, the end point of the heat treatment can be specified, forexample, as the point where heating the product at 1,800° C. for onehour causes a mass reduction of less than 1% by mass.

Heat treatment of a silicone cured product obtained by first molding thecurable silicone composition into a desired shape of desired dimensionsand subsequently curing the composition may be conducted eitherfollowing removal of the silicone cured product from the molding die inthe case of a metal mold, or without removing the cured product in thecase of a sand mold.

EXAMPLES

A more detailed description of the present invention is presented belowbased on a series of examples, although the present invention is in noway limited by the examples described below. In the examples, molecularweight values are weight average molecular weight values measured usingGPC and referenced against polystyrene standards. Further, the averageelemental ratio between the compositional elements within a productproduced by heat treatment is simply referred to as the “elementalratio”, and is represented by an elemental composition formula: SiC_(g)(wherein, g is a number of 0 or greater). This elemental compositionformula indicates that the average elemental ratio between silicon andcarbon within the product is 1:g. Moreover, “Me” represents a methylgroup.

Example 1

The components (A) and (B) described below were used as siliconecomponents, and the component (C) described below was used as a platinumgroup metal-based catalyst. The quantity of each component is also shownbelow.

(A) 55 parts by mass of a diorganopolysiloxane containing alkenyl groupsbonded to silicon atoms, represented by an average composition formula(5) shown below.

(B) 45 parts by mass of a diorganopolysiloxane containing hydrogen atomsbonded to silicon atoms, represented by an average composition formula(6) shown below (wherein, the quantity of SiH groups within thecomponent (B) per 1.0 mols of silicon atom-bonded alkenyl groups withinthe component (A) is 1.0 mols).

(wherein, the molar ratio of SiH groups relative to all the siliconatoms within the molecule is 0.625).(C) a toluene solution of a platinum-divinyltetramethyldisiloxanecomplex (platinum element content: 0.5% by mass), in a quantityequivalent to 50 ppm of the platinum element relative to the mass of thecomponent (A).

The above components (A) and (B) were placed in a planetary mixer (aregistered trademark for a mixing device manufactured by InoueManufacturing Co., Ltd.), and were stirred for one hour at roomtemperature. Subsequently, the component (C) was added to the planetarymixer and stirring was continued for a further 30 minutes at roomtemperature, thus yielding a curable silicone composition with aviscosity at room temperature of 50 mPa·s. This curable siliconecomposition was cured by heating at 80° C. for one hour.

The thus obtained silicone cured product was placed in a vessel formedof carbon that was subsequently placed inside an atmosphere furnace, andunder an atmosphere of argon gas, the temperature was raised to 1,800°C. over an 18-hour period at a rate of temperature increase of 100°C./hour. The temperature was then held at 1,800° C. for two hours, andthen cooled to room temperature, yielding a yellow-green colored solid.When this yellow-green solid was subjected to a carbon analysis using aCS-444LS analyzer (a product name, manufactured by LECO Corporation),the carbon mass ratio was 30.3% by mass. Further, when the yellow-greensolid was subjected to an oxygen analysis using a TC436 analyzer (aproduct name, manufactured by LECO Corporation), the oxygen mass ratiowas not more than 0.2% by mass. Measurement of the elemental ratio forthe yellow-green solid by EDX analysis (Energy Dispersive X-rayanalysis) using a FE-SEM (Field Emission Scanning Electron Microscope)yielded a result of SiC_(1.02). Moreover, measurement of theyellow-green solid using an X-ray diffraction method yielded the X-raydiffraction spectrum shown in FIG. 1( a). Comparison of the peak datafor this spectrum (FIG. 1( b)) with the peak data for the X-raydiffraction spectrum of β-silicon carbide crystals (FIG. 1( c)) revealeda good match for the two sets of data, confirming that the yellow-greensolid described above was composed of β-silicon carbide crystals.

Example 2

100 parts by mass of a silicone resin containing only MeSiO_(3/2) unitsas the siloxane units and also containing 5% by mass of hydroxyl groups(molecular weight: 1,000, average composition formula:Me(OH)_(0.2)SiO_(1.3), softening point: 65° C.) was placed in analuminum Petri dish and cured by heating at 200° C. for one hour. Theresulting silicone cured product was subjected to a heat treatment inthe same manner as Example 1, yielding a yellow-green solid. Analysis ofthis yellow-green solid in the same manner as that described in Example1 revealed a carbon mass ratio of 30.4% by mass, an oxygen mass ratio ofnot more than 0.2% by mass, and an elemental ratio of SiC_(1.02).

Example 3

The components (A) to (C) used in Example 1 and a component (D)described below as a silicon carbide powder were used in the respectivequantities listed below.

(A) 55 parts by mass of the diorganopolysiloxane containing alkenylgroups bonded to silicon atoms and represented by the averagecomposition formula (5) shown above.(B) 45 parts by mass of the diorganopolysiloxane containing hydrogenatoms bonded to silicon atoms and represented by the average compositionformula (6) shown above.(C) a toluene solution of a platinum-divinyltetramethyldisiloxanecomplex (platinum element content: 0.5% by mass), in a quantityequivalent to 0.15% by mass relative to the combined mass of thecomponent (A) and the component (B).(D) 327 parts by mass of a silicon carbide powder (volume averageparticle size: 10 μm) obtained by pulverizing the yellow-green solid ofExample 1 using a ball mill (this quantity is equivalent to the quantityrequired to ensure that the silicon carbide powder represents 50% byvolume of the entire silicone composition at room temperature).

The above components (A), (B) and (D) were placed in a planetary mixer(a registered trademark for a mixing device manufactured by InoueManufacturing Co., Ltd.), and were stirred for one hour at roomtemperature. Subsequently, the component (C) was added to the planetarymixer and stirring was continued for a further 30 minutes at roomtemperature, thus yielding a curable silicone composition with aviscosity at room temperature of 3,000 mPa·s. Subsequently, 75 g of thiscurable silicone composition was poured into a screen mask of thickness4 mm having an opening of 130 mm×190 mm. The composition was then heatedin the air at 125° C. for one hour, thus forming a silicone curedproduct. The silicone cured product was removed from the screen mask andthen subjected to a heat treatment in the same manner as Example 1,yielding a yellow-green solid (dimensions: 130 mm×190 mm×4 mm). Analysisof this yellow-green solid in the same manner as that described inExample 1 revealed a carbon mass ratio of 30.3% by mass, an oxygen massratio of not more than 0.2% by mass, and an elemental ratio ofSiC_(1.02).

1. A method of producing silicon carbide, comprising heating a curedproduct of a curable silicone composition in a non-oxidizing atmosphereat a temperature exceeding 1,500° C. but not more than 2,600° C.,wherein prior to the step of heating the cured product, the curablesilicone composition is molded into a desired shape of desireddimensions and then cured to obtain the cured product, so that thesilicon carbide is produced in the desired shape of desired dimensionsafter heating said cured product in the non-oxidizing atmosphere at saidtemperature exceeding 1,500° C. but not more than 2,600° C., wherein thecurable silicone composition is a condensation-curable siliconecomposition comprising: (α) a silicone resin represented by an averagecomposition formula (3) shown below:R³ _(m)R⁴ _(n)(OR⁵)_(p)(OH)_(q)SiO_((4-m-n-p-q)/2)  (3) (wherein, eachR³ represents, independently, a hydrogen atom or a monovalenthydrocarbon group other than an aryl group that either comprises or doesnot comprise a carbonyl group, R⁴ represents a phenyl group, R⁵represents a monovalent hydrocarbon group of 1 to 4 carbon atoms, mrepresents a number that satisfies: 0.1≦m≦2, n is 0, p represents anumber that satisfies: 0≦p≦1.5, and q represents a number thatsatisfies: 0≦q≦0.35, provided that a value of m+n+p+q satisfies:0.1≦m+n+p+q≦2.6, (β) optionally, a hydrolysable silane or a partialhydrolysis-condensation product thereof or a combination thereof, and(γ) optionally, a condensation reaction catalyst.
 2. The method ofproducing silicon carbide according to claim 1, wherein the curablesilicone composition further comprises a silicon carbide powder.
 3. Themethod of producing silicon carbide according to claim 1, wherein R³ inthe composition formula (3) is a methyl group.